High input impedance low output impedance amplifier

ABSTRACT

An amplifier circuit with high input impedance and low output impedance intended for being supplied with current via the output signal line includes a pre-amplifier with a transistor of the field-effect type (Q12) and a bipolar transistor (Q13) directly connected thereto, where the base-emitter circuit of the bipolar transistor and the source-drain circuit of the field-effect transistor (Q12) are connected in series across the supply source. The gate of the field-effect transistor is connected to a biasing source (16), the temperature coefficient of which has substantially the same absolute value as the temperature coefficient of the base-emitter voltage in the bipolar transistor (Q13), but with reverse sign.

TECHNICAL FIELD

The invention relates to an amplifier circuit with high input impedanceand low output impedance, preferably intended as a microphone amplifierin telephone apparatus with electret microphones and for being suppliedwith current via the telephone line.

BACKGROUND ART

Electret microphones are used to an ever-increasing extent in moderntelephone apparatus, since they give good sound quality, have low powerconsumption and are inexpensive in manufacture. As with allelectrostatic microphones, the electret microphone has a very highimpedance and low output power, so an amplifier must therefore beconnected between the microphone and the line. Due to the electretmicrophone's high impedance, this amplifier should be immediatelyadjacent the microphone to avoid circuit noise. Suitably, the microphoneand amplifier are built together to a capsule which is placed in thetelephone handset. Current supply then suitably takes place using thesame conductor as for the signals from the handset to the telephone set.A suitable circuit for connecting a source having very high impedance toa load with low impedance suitably contains an input stage with afield-effect transistor directly connected to one or more emitterfollower stages. Such circuits are known in the literature e.g. from"Electronics" Feb. 28, 1972, page 80, which illustrates an amplifier fora capacitive transducer.

DISCLOSURE OF INVENTION

When an amplifier of this kind is to be used as a microphone amplifier,large demands are placed on it for operation at low voltages (lower than2 volts) and within a large temperature range (-20°-+70° C.) with lowdistorsion.

For the field-effect transistor to work linearly at constant bias overthe whole temperature range the bias must be made high, due to thethreshold voltage increasing with the temperature. At low temperatures,the sum of this voltage and the voltage, increasing with fallingtemperature, of the base-emitter voltage in the emitter follower stagewill then be so large that there is the risk of the voltage of theamplifier current supply circuit not being sufficient. The field-effecttransistor has, however, the property that it does not need such a highvoltage at low temperatures, due to the threshold voltage having apositive temperature coefficient.

In accordance with the invention, this relationship can be utilized foroptimally utilizing available supply voltage in an amplifier circuitwith one input stage having at least one field-effect transistordirectlyg connected to a bipolar amplifier stage. The characterizingfeatures of the invention are disclosed in the accompanying claims.

DESCRIPTION OF FIGURE

The invention will now be described in detail in conjunction with theappended drawing whose sole FIGURE, illustrates a circuit diagram for anelectret microphone amplifier.

PREFERRED EMBODIMENT

An integrated amplifier, denoted by the numeral 11 in the sole FIGURE,has only three exterior connections, namely 12, which is a common signaloutput and positive voltage supply, a common point 13 for the amplifierinput and output (earth) as well as negative voltage supply, and thesignal input 14. An electret microphone M is connected between thesignal input 14 and earth 13. The integrated circuit is built upconventionally with a p substrate and an epitaxial n layer, whereintransistors and other elements have been made using the diffusiontechnique.

The input stage of the amplifier consists of a field-effect transistorQ12 of the P channel type, with its gate connected to the input 14. Thistransistor Q12 is fed with current from a current mirror circuitincluding the transistors Q1, Q2 and the field-effect transistor Q3. Thefield-effect transistor Q3, which is exactly equal to the field-effecttransistor Q12, is connected as a constant current generator supplyingthe diode-connected transistor Q1. The current in transistor Q1 isreflected to the identically similar transistor Q2 driving the samecurrent through the field-effect transistor Q12. The transistor Q2 shalloperate linearly.

The drain of the field-effect transistor Q12 is directly connected tothe base of the emitter follower stage Q13, the collector of which getsits current supply from the field-effect transistor Q14. The collectorof the transistor Q13 is directly connected to the base of the n-p-ntransistor Q15, the emitter-collector circuit of which is directlyconnected to the supply lines 12, 13. The transistors Q13 and Q14 may beconsidered together as an emitter follower stage with high currentamplification.

The gate of the field effect transistor Q12 is connected via a very highresistance R to the output 15 from a reference voltage source 16 of thebandgap reference type. The bandgap reference source 16 consists of thetransistors Q4, Q5, Q6, Q7 and Q8 conventionally disposed in a currentmirror circuit. The transistors Q7 and Q8 are of different sizes andtherefore have different current densities for the same current. In theembodiment, Q7 is the transistor which has the greatest area. Thetransistors Q4, Q5 and Q6 are of the same size, and with the aid of thecurrent mirror they carry the same current. Due to the lower currentdensity in the transistor Q7, this will have a lower base-emittervoltage. The difference in the base-emitter voltage between thetransistors Q7 and Q8 is obtained across the resistor R3 in the emittercircuit.

The reference current of the bandgap reference gives rise to an equallyas great current through the transistor Q6, the collector of which isconnected to earth through the resistors R5 and R4. The voltage dropacross these resistors is taken out as the reference voltage at thepoint 15 and constitutes the bias to the field-effect transistor Q12 inthe input stage of the amplifier. When the voltage supply is connectedto the bandgap reference source 16, no current flows to either of thetransistors Q7 and Q8, and it can therefore not be excited. In order toenable excitation, the transistor Q9 is connected in parallel with thetransistor Q7 and the resistor R3. The base of the transistor Q9 isconnected to the collector of the transistor Q13. At the connectioninstant the transistor Q13 momentarily has the base voltage zero and isheavily conducting. The base of the transistor Q15 becomes positive andthe transistor Q9 will be conducting. The transistor Q4 will beconducting and current begins to flow in the bandgap reference source16. The potential drop across resistor R4 cuts off transistor Q9.

As mentioned above, the emitter of the field-effect transistor Q12 willobtain a voltage falling by a base-emitter potential drop below thesupply voltage on the supply line 12. The same voltage also appears atthe gate of Q12 due to the current mirror circuit Q3, Q1, Q2. Theidentical field-effect transistor Q3 namely has source and gateinterconnected.

The base-emitter voltage of a bipolar transistor has a temperaturecoefficient of -2 mV/°C. With the present circuit, where the amplifierhas to operate between -20° C. and +70° C., it is required that thebase-emitter voltage is considerably raised in the bipolar transistorQ13 to enable maintenance of the collector current, and thereby theamplification even at low temperatures. This means that the field effecttransistor Q12 must be satisfied with a lower source-drain voltage, ifthe total supply voltage across the circuit cannot increase. Afield-effect transistor has, however, a positive temperature coefficientfor the pinch-off voltage Vp. It is therefore possible to dimension thebias at point 15 so that it obtains a positive temperature coefficient.By suitable selection of this positive temperature coefficient, it ispossible to utilize the properties of the field effect transistor suchthat both amplifier stages are given substantially constantamplification at acceptably low distortion over the entire temperaturerange. At low temperatures the transistor Q13 obtains a higherproportion of the available supply voltage, while the field-effecttransistor, which has better data at lower temperatures, must besatisfied with a lower proportion. The reverse condition applies athigher temperatures.

The temperature control of the bias at point 15 is provided by selectingthe temperature coefficient of the badgap reference source 16. Here thetemperature coefficient is determined by the temperature coefficient ofthe resistor R3, plus the temperature coefficient of the difference inthe base-emitter voltage between the transistors Q8 and Q7. Theseparameters are suitably set such that the temperature coefficient of thevoltage at point 15 is of the same order of magnitude as for thebase-emitter voltage in the transistor Q13, but with reversed sign, i.e.about +2 mV/°C.

I claim:
 1. An amplifier circuit powered by a low supply voltage sourceincluding an amplifier with a field-first effect transistor, a bipolartransistor and means for interconnecting the first field-effecttransistor and the bipolar transistor to the low supply voltage sourcein such a manner that the base-emitter circuit of the bipolar transistorand the source-drain circuit of the first field-effect transistor areconnected in series across the low supply voltage source, theimprovement comprising that the first field-effect transistor isincluded in the first branch of a current mirror circuit also having anidentical second branch which is a current generator branch including asecond field-effect transistor identical to said first field-effecttransistor, the gate of said second field-effect transistor beingdirectly to the source of said second field-effect transistor, wherebythe drain of said first field-effect transistor always follows the gateof said second field-effect transistor, and that the gate of the firstfield-effect transistor is connected to a biasing means having atemperature coefficient with the same absolute value but opposite insign as the temperature coefficient of the base-emitter voltage of thebipolar transistor.
 2. The amplifier circuit of claim 1 wherein thebiasing source comprises a bandgap reference voltage source.